I. Field of the Invention
This invention relates generally to medical diagnostic apparatus, and more particularly to indicator circuitry included in an electrocardiograph (ECG) system for indicating whether one or more skin electrodes is properly connected to a patient.
II. Discussion of the Prior Art
Electrocardiography is the recording, usually from electrodes on the body surface, of the electrical activity of the heart during the cardiac cycle. Before each part of the heart contracts, there is a change in the membrane potential of the cardiac muscle cells, thus depolarization precedes contraction while repolarization follows and precedes relaxation. The potential differences can be recorded from electrodes on the body surface and the appearance of the recorded ECG depends on the sequence of depolarization and repolarization of the cardiac muscle mass and the position of the recording electrodes. A typical ECG utilizes 12 leads, such that 12 samples may be recorded with standard connections between the patient and the ECG machine. Interpretation of an ECG can provide a very detailed picture of heart function but, obviously to do this, requires considerable skill and experience. To properly understand and interpret ECG recordings, one must be able to understand the origin of the cardiac vectors, know the axes of the 12 ECG leads and appreciate the convention of normal vectors of depolarization and repolarization.
At rest, during diastole, the resting membrane potential cannot be detected without puncturing the cell with a microelectrode in that it does not cause any current to flow in the extracellular fluid. When the cardiac action potential propagates through the tissue, current flows in the extracellular fluid and the intracellular fluid, driven by the difference between membrane potentials in resting and depolarized zones. The potential difference recorded is a vector quantity in that it has both magnitude and direction and conventionally may be represented by an arrow pointed towards the resting membrane, i.e., in the direction of spread of electrical activity. The length of the arrow, of course, indicates the magnitude of the potential. By convention, if the electric vector is oriented toward the positive recording electrode, it is represented by an upward deflection of the ECG.
A triangle (Einthoven's Triangle) with the heart at its center is formed by placing recording limb electrodes on both arms and the left leg. The Einthoven's Triangle is generally represented as an equilateral in that the trunk of the patient is a uniform volume conductor and the heart acts a point source of electric vectors situated at its center. Vector I is defined as the potential difference between the right arm (RA) electrode and the left-arm (LA) electrode. Vector II is the potential difference between the RA electrode and the left-leg (LL) electrode. Vector III is the potential difference between the LA and the LL electrodes. According to Einthoven's Law, only two such leads are independent in that the third lead can be simply calculated from the other two.
An ECG system also employs chest leads. More particularly, in clinical routine use, six chest leads are used to record cardiac events under a single electrode with respect to an “indifferent” electrode. This reference point is formed by connecting the RA, LA and LL electrodes together with resistors, with the reference potential being appropriately the middle point of the Einthoven's Triangle, sometimes referred to as the Wilson Potential. Three additional vectors referred to as “augmented limb lead vectors” are based upon the Wilson Potential. The three limb electrodes, the six chest electrodes and three augmented limb electrodes total the twelve leads.
Many ECG machines incorporate a “lead-off indictor” to help identify a high-impedance ECG electrode patch. By providing such an indicator, a medical professional is able to quickly locate the source of a noisy signal and take appropriate steps to secure the lead patch to the skin of the patient. This reduces the amount of set-up and trouble-shooting time involved with an ECG measurement. Most conventional leads-off indicators use simple impedance measurements to determine whether an electrode is attached to the patient. Typically, the ECG machine applies a relatively high frequency (e.g., 30 KHz) drive signal to the patient through the electrode affixed to the patient's right-leg (RL) electrode. The ECG machine then measures this signal through the other input electrodes to determine whether the electrodes are properly attached by comparing the amplitude of the transduced 30 KHz signal to a predetermined reference.
This conventional approach of applying a high frequency drive signal to the RL electrode has drawbacks when several medical devices are used in conjunction with a given patient. Often several ECGs and monitors are connected to the patient at once, potentially causing errors in the leads-off indication if several such machines utilize the standard 30 KHz excitation signal. This problem can be significantly worse with certain pacemaker patients. Pacemakers from several manufacturers also utilize an excitation frequency near 30 KHz for deriving a rate-adaptive control signal based upon minute ventilation. The 30 KHz drive signal is applied by the pacemaker pulse generator circuitry as a carrier signal that is modulated by respiratory activity. The modulation signal is proportional to minute ventilation which is a parameter that varies predictably with the level of patient activity.
When ECG machines with prior-art style leads-off indicators are used with pacemaker patients having a minute ventilation-based rate adaptive pacemaker, the leads-off indicator can cause pacing at the upper-rate limit. In October 1998, the FDA's Center for Devices and Radiological Health issued an alert, warning physicians of this interaction.
In addition to the affects on the device, telemetry interference at 30 KHz can be significant and may cause the leads-off indicator to provide erroneous results. Thus, a need exists for a leads-off indicator for use in ECG equipment that can operate without interference from or with other medical devices being used with a given patient.
The present invention provides a leads-off indicator that does not require a 30 KHz excitation signal, but instead, utilizes information from the common-mode input noise to determine whether an electrode is connected to the patient. Nearly all ECG equipment operates in electrical environments with high levels of power line noise, 60 Hz being the dominant common-mode signal on the input electrodes for equipment used in the United States and 50 Hz in Europe. By comparing the relative noise between vectors, the RL output, and the common-mode input voltage, the noise level can be triangulated to reveal a high impedance electrode. By using the common-mode input noise instead of a 30 KHz drive signal, compatibility of the ECG leads-off indicator with minute ventilation-based rate adaptive pacemakers is provided, as is a high immunity to interference from telemetry or other monitors.